1. Field of Invention
This invention relates to an illuminating optical device for uniformly illuminating an object, which is suitably used in exposure apparatuses for manufacturing highly integrated semiconductor circuits.
2. Description of Related Art
One example of conventional illuminating optical devices is shown in FIG. 5, which is designed to uniformly illuminate objects. The light flux emitted from the light source 1 is shaped by the light shaping optical system so as to fit the fly-eye lens 3. The shaped light flux is divided into a plurality of light components by the fly-eye lens 3, and a plurality of secondary light-source images are formed in the vicinity of the exit plane of the fly-eye lens 3. An aperture stop 4 is positioned on or near the secondary light-source image plane in order to limit the light flux from the plurality of secondary light-source images. The light flux is collected by the condenser lens 5, and guided onto the mask 6 in such a manner that the fluxes from the multiple secondary light-source images overlap, thereby uniformly illuminating the pattern on the mask 6. The pattern image is projected onto the wafer surface 8 by the projection lens system 7.
Because the light flux is divided into multiple components by the fly-eye lens 3, and because the condenser lens 5 guides these light components onto the mask 6 so that they overlap each other when they illuminate the pattern, the light intensity becomes satisfactorily uniform on the wafer 8 even if the intensity distribution of the original light flux from the light source 1 is not perfectly uniform.
Recently, a technique for varying the aperture pattern of the aperture stop 4 in accordance with the mask pattern to be projected onto the wafer 8 has drawn a great deal of attention because this technique can further improve the resolution and the focal depth. This technique is called variable illumination. Some examples of aperture patterns of an aperture stop are illustrated in FIG. 4.
However, it was found through various analyses and experiments that if the aperture pattern of the aperture stop 4 is changed during the illumination of an optical system, the illuminance becomes uneven on the wafer 8. This unevenness of illuminance causes the thickness of the circuit pattern image projected onto the wafer 8 to vary depending on the position on the wafer 8, which variance adversely affects the performance of the resultant semiconductor circuit. FIGS. 6 and 7 illustrate typical types of unevenness in illuminance. The illuminance distribution having uneven on wafer includes convex component as shown FIG. 6 (or concave component) and sloped component as shown FIG. 7. In FIG. 6, the illuminance distribution is convex, while, in FIG. 7, the illuminance distribution is sloped. There are many factors, which may combine with each other, which cause the uneven illuminance, for example, manufacturing error in the optical system, decentering of the aperture stop, non-uniform distribution of transmissivity of the optical system (due to the characteristic of the AR coat or dust), etc.